Project description:Cyclin C was cloned as a growth-promoting G1 cyclin1,2, and several studies postulated a role for cyclin C in driving cell proliferation3-8 . Moreover, cyclin C, together with its kinase partner, the cyclin-dependent kinase CDK8, is believed to represent an essential component of basal transcriptional machinery where it globally represses gene expression9-13. However, the function of cyclin C in vivo has never been addressed. Here we show that in the living organism cyclin C acts as a haploinsufficient tumor suppressor, through its function of controlling Notch1 oncogene levels. Cyclin C activates an “orphan” CDK19 kinase14, as well as CDK8 and CDK3. These cyclin C-CDK complexes phosphorylate Notch1 intracellular domain (ICN1), which allows binding of ICN1 to Fbw7 and triggers ICN1 polyubiquitination. Genetic ablation of cyclin C blocks ICN1 phosphorylation, disrupts Fbw7 binding, and decreases ICN1 ubiquitination in vivo, thereby strongly elevating ICN1 levels in several compartments of cyclin C knockout mice. Cyclin C was cloned as a growth-promoting G1 cyclin1,2, and several studies postulated a role for cyclin C in driving cell proliferation3-8 . Moreover, cyclin C, together with its kinase partner, the cyclin-dependent kinase CDK8, is believed to represent an essential component of basal transcriptional machinery where it globally represses gene expression9-13. However, the function of cyclin C in vivo has never been addressed. Here we show that in the living organism cyclin C acts as a haploinsufficient tumor suppressor, through its function of controlling Notch1 oncogene levels. Cyclin C activates an “orphan” CDK19 kinase14, as well as CDK8 and CDK3. These cyclin C-CDK complexes phosphorylate Notch1 intracellular domain (ICN1), which allows binding of ICN1 to Fbw7 and triggers ICN1 polyubiquitination. Genetic ablation of cyclin C blocks ICN1 phosphorylation, disrupts Fbw7 binding, and decreases ICN1 ubiquitination in vivo, thereby strongly elevating ICN1 levels in several compartments of cyclin C knockout mice.

Project description:Cyclin C was cloned as a growth-promoting G1 cyclin, and several studies postulated a role for cyclin C in driving cell proliferation. Moreover, cyclin C, together with its kinase partner, the cyclin-dependent kinase CDK8, is believed to represent an essential component of basal transcriptional machinery where it globally represses gene expression. However, the function of cyclin C in vivo has never been addressed. Here we show that in the living organism cyclin C acts as a haploinsufficient tumor suppressor, through its function of controlling Notch1 oncogene levels. Cyclin C activates an M-bM-^@M-^\orphanM-bM-^@M-^] CDK19 kinase, as well as CDK8 and CDK3. These cyclin C-CDK complexes phosphorylate Notch1 intracellular domain (ICN1), which allows binding of ICN1 to Fbw7 and triggers ICN1 polyubiquitination. Genetic ablation of cyclin C blocks ICN1 phosphorylation, disrupts Fbw7 binding, and decreases ICN1 ubiquitination in vivo, thereby strongly elevating ICN1 levels in several compartments of cyclin C knockout mice. Ablation of cyclin C, or cyclin C heterozygosity collaborate with other oncogenic lesions and accelerate development of T-cell acute lymphoblastic leukemia (T-ALL) in cyclin Cdeficient mice. Furthermore, the locus encoding cyclin C is heterozygously deleted in a significant fraction of human T-ALL, and these tumors express reduced cyclin C levels. In addition, we describe point mutations in human T-ALL tumors that render cyclin C-CDK unable to phosphorylate ICN1. These studies reveal that in sharp contrast to all other cyclin proteins, cyclin C functions as a growth-suppressor in vivo, and suggest that human tumor cells develop different strategies to evade cyclin C inhibitory function. Comparison of wild-type mouse embryonic fibroblasts (n=3 biological replicates) versus cyclin C knockout MEFs (n=3), wild-type mouse embryonic stem cells (n=3) versus cyclin C knockout ESC (n=3), wild-type mouse embryonic brain (n=3) versus cyclin C knockout embryonic brain (n=3)

Project description:Cyclin C was cloned as a growth-promoting G1 cyclin1,2, and several studies postulated a role for cyclin C in driving cell proliferation3-8 . Moreover, cyclin C, together with its kinase partner, the cyclin-dependent kinase CDK8, is believed to represent an essential component of basal transcriptional machinery where it globally represses gene expression9-13. However, the function of cyclin C in vivo has never been addressed. Here we show that in the living organism cyclin C acts as a haploinsufficient tumor suppressor, through its function of controlling Notch1 oncogene levels. Cyclin C activates an “orphan” CDK19 kinase14, as well as CDK8 and CDK3. These cyclin C-CDK complexes phosphorylate Notch1 intracellular domain (ICN1), which allows binding of ICN1 to Fbw7 and triggers ICN1 polyubiquitination. Genetic ablation of cyclin C blocks ICN1 phosphorylation, disrupts Fbw7 binding, and decreases ICN1 ubiquitination in vivo, thereby strongly elevating ICN1 levels in several compartments of cyclin C knockout mice. Cyclin C was cloned as a growth-promoting G1 cyclin1,2, and several studies postulated a role for cyclin C in driving cell proliferation3-8 . Moreover, cyclin C, together with its kinase partner, the cyclin-dependent kinase CDK8, is believed to represent an essential component of basal transcriptional machinery where it globally represses gene expression9-13. However, the function of cyclin C in vivo has never been addressed. Here we show that in the living organism cyclin C acts as a haploinsufficient tumor suppressor, through its function of controlling Notch1 oncogene levels. Cyclin C activates an “orphan” CDK19 kinase14, as well as CDK8 and CDK3. These cyclin C-CDK complexes phosphorylate Notch1 intracellular domain (ICN1), which allows binding of ICN1 to Fbw7 and triggers ICN1 polyubiquitination. Genetic ablation of cyclin C blocks ICN1 phosphorylation, disrupts Fbw7 binding, and decreases ICN1 ubiquitination in vivo, thereby strongly elevating ICN1 levels in several compartments of cyclin C knockout mice.

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Cyclin C is a haploinsufficient tumour suppressor

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